In an example conventional radio frequency (RF) power amplifier (PA), a transistor receives at its control terminal an RF signal, and it amplifies the RF signal for the load. The load may include a 50 ohm antenna, a duplexer, and or other appropriate components. Continuing with the example, the PA includes a matching network between the transistor and the load, so that a node at the drain of the transistor sees a relatively small impedance, e.g., 3-4 ohms. However, the impedance at the load may change. In an example where the load impedance is 50 ohms, the load impedance may vary from 25 ohms to 75 ohms during normal use.P=(V2)/(2R)  Equation 1
Equation 1 above provides an insight into how the PA operates. P is the output power, V is the voltage swing or AC amplitude at the drain of the transistor, and R is the voltage seen by the node at the drain of the transistor. The matching network matches the impedance so that the impedance seen by the node at the drain of the transistor may be relatively small, and if the output power P is held constant V should also be relatively small. However, insertion loss by the matching network increases as the impedance seen by the node decreases. This hurts the power added efficiency (PAE) of the amplifier. Also, raising the impedance seen by the node would also raise the AC voltage swing, which would decrease linearity. An example measure of linearity includes adjacent channel leakage ratio (ACLR). Therefore, a conventional PA faces the trade-off between efficiency and linearity.
Maintaining efficiency and linearity becomes complicated when, as mentioned above, the impedance at the load may vary during normal use. For instance, if the impedance at the load goes to 75 ohms, that causes the impedance seen by the node to go down and the AC voltage swing to get smaller. Similarly, if the impedance at the load decreases to 25 ohms, that causes the impedance seen by the node to go up and the AC voltage swing to increase.
It would be desirable to make adjustments of the PA system to maintain AC voltage swing and linearity within a desired range, even as impedance at the load may change.